Thermally Activated Exciton Transition Induced Long Photoluminescence Lifetime in Large CsPbBr3 Nanocrystals.

Large CsPbBr3 nanocrystals show unique photoluminescence properties of an ultralong lifetime, supernarrow line width, and high exciton efficiency; however, the mechanism of the ultralong photoluminescence lifetime is still not clear. In this work, we investigated the size- and temperature-dependent photoluminescence properties of CsPbBr3 nanocrystals with steady and transient photoluminescence spectra. At cryogenic temperature, small CsPbBr3 nanocrystals (8.6 and 10.6 nm) show a nearly single-peak emission of confined excitons, while large CsPbBr3 nanocrystals (12.9, 17.5, and 25.8 nm) exhibit two band emissions with a short component lifetime (several nanoseconds) of free excitons and a long component lifetime (hundreds of nanoseconds) of bound excitons. For large CsPbBr3 nanocrystals, the ratio of photoluminescence emission between bound and free excitons varied from 7:3 to 3:7 with increasing temperature. Based on the experimental results, we propose a mechanism of thermally activated transition between bound and free excitons to explain the ultralong photoluminescence lifetime in large CsPbBr3 nanocrystals. In all, this understanding may exploit these thermally activated transition effects to design advanced quantum emitters.